Implantable medical devices (IMDs), including pacemakers and implantable cardioverter-defibrillators (ICDs), record cardiac electrogram (EGM) signals for detecting the heart rhythm and responding as needed with pacing therapy or high-voltage cardioversion/defibrillation therapy. Numerous criteria may be applied to the EGM signals for detecting arrhythmias and for discriminating between different types of arrhythmias, such as supraventricular tachycardia (SVT), ventricular tachycardia (VT) and ventricular fibrillation (VF). Forms of SVT, including sinus tachycardia, atrial fibrillation and atrial flutter, can be referred to as “non-treatable” or “non-shockable” rhythms in that typically a cardioversion/defibrillation shock delivered to the heart is undesirable for treating these more benign rhythms. Sustained VT and VF, on the other hand, can be referred to as “treatable” or “shockable” rhythms because such sustained rhythms are more serious and potentially life-threatening. Detection of a sustained VT or VF is generally treated by anti-tachycardia pacing (ATP) or a cardioversion/defibrillation shock. One important goal of a tachyarrhythmia detection algorithm is to detect all treatable VT and VF episodes.
Another goal is to avoid delivering a shock therapy when a fast rhythm is a non-treatable rhythm. SVT is sometimes inappropriately detected as VT or VF when the SVT rate falls in a VT or VF rate zone (i.e. therapy zone). The inappropriate VT or VF detection can result in the patient receiving an unnecessary cardioversion/defibrillation shock. Therefore, what is needed is a system and method that provides reliable detection and discrimination of tachyarrhythmias and reduces the likelihood of inappropriate VT or VF detections in the presence of SVT.